Frequency multiplier



April 12, 1949. R. H. VARIAN 2,466,754

FREQUENCY MULTIPLIER Original Filed March 23 1940 wwwwm,

INVENTOR RUSSELL H. VARIAN ATTORNEY Patented Apr. 12, 1949 UNITED .s'rA'rss PATENT OFFICE 2,466,754 FREQUENCY MULTIPLIER Russell H. Varian, Garden City, N. Y., assignor to The Board of Trustees of The Leland Stanford Junior University, Stanford University,

Calif.. a corporation-of California Application March 23, 1940, Serial No. 325,634, .which is a division of application Serial No. 168,355, October 11, 1937. Divided and this application October 24, 1944', Serial No.-560,147

1' The present invention relates to electrical translating systems and methods and, more particularly, to systems and methods employing electrical discharges. The present application is a division of my pending application Serial No. 325,634, filed March 23, 1940, which in turn is a true division of my application Serial No. 168,355, filed October 11, 1937, now Patent No. 2,242,275, granted May 20, 1941. v

The, present invention utilizes a completely novel principle of electron beam control for ultrahigh-frequency frequency-multiplication. Adaptation of the same principle to the problem of ultra-high-frequency amplification and generation of oscillations is claimed in the above-mentioned prior applications serial Nos. 168,355 and 325,634, respectively. According to this principle, instead of utilizing the conventional input grid and impressing the input signal or voltage between this grid and cathode as has been customary in the electron beam art, a pair of electrodes are provided along the electron stream in controlling relation thereto. The input signal is impressed between these. two electrodes to produce a controlling alternating electric field along the direction of the electron flow. The electron stream is generated independently of this ontrol action impressed thereon. This control action alternately accelerates and decelerates electrons of the stream at the periodicityof the input sighad. The continuing flow of these electrons beyond the influence of the controlling electrodes causes the previously uniform electron stream to become a variable current or density electron stream by the action of the faster '0! accelerated electrons overtaking the slower or decelerated electrons. The electron stream is thereby caused to vary periodically in electron density and current intensity at the frequency of the input signal and also at several harmonics thereof. According to a principal feature of the invention, this varying electron stream current is caused to excite an output circuit at a desired harmonic of the input frequency. This output circuit excitation is performed completely inductively by having the electron stream pass between a further pair of electrodes in controlling but not intercepting relation to the stream. A suitable output circuit is connected to these further electrodes, andit is preferably tuned to the desired harmonic frequency output. Passage of the varying electron current through and between these electrodes then induces harmonic frequency energy in the output circuit, which 13 Claims. (Cl. 315-6) energy may be usefully extracted and supplied to any desired load.

Accordingly, the principal object of the present invention is to provide a new and improved method of and apparatus for frequency multiplication operable with high efiiciency at ultra high frequencies, including wavelengths of the order of ten centimeters or smaller.

Further objects and advantages of the present invention will become apparent from the following specification and the accompanying drawings, in which:

Fig. 1 is a schematic longitudinal sectional view of an amplifier and frequency-multiplier device forming the present invention; and

Fig. 2 is a fragmentary view, partly in section and partly in perspective, showing a portion of the toroidal resonant circuit or resonator utilized in the device shown in Fig. 1.

Utilizing the principles of the present invention, it is possible to build amplifying tubes having a high power output at ultra high frequencies which may be represented by wavelengths, for example, of ten centimeters or less; and there is herein described an amplifier and frequency multiplier wherein all of the resonant circuits are small enough to be self-contained within a single envelope. In this respect there is employed as a basic principle for the design of the amplifying or frequency-multiplying circuits at confined-field resonator in theform of a hollow, metal, internally space-resonant or tuned body. Such resonators have come to be called hollow or cavity resonators.

To illustrate my invention, I have shown in the drawings a frequency multiplier structure embodying my invention in order that those skilled in the art may be able from the disclosure to build other apparatus following the same broad principles as herein contained.

The word grid, as utilized herein, refers to any electron-permeable electrode, irrespective of the number of apertures present, so placed as to influence electrons or to be influenced by them.

Referring to Fig. 1, which shows a thermionic tube or apparatus connected to control an electron stream, an envelope I is provided at one end thereof with a reentrant stem 2 which supports a thermionic cathode 4. It is obvious that any of the known thermionic structures, directly or indirectly heated, may be utilized to form the source of electrons for the tube. The electrons emitted from the cathode 4 are caused to travel therefrom in the form of a stream along a substantially straight-line path, under the influence of the electric field between cathode 4 and an accelerating electrode 20, produced by an accelerating battery 6 whose negative terminal is connected to cathode 4 and whose positive terminal is grounded. Along this path the electrons travel through the electrode 29, into and through a cavity resonator or space-resonant device 2| of the above-described character, through a space or region 24 and through a second cavity resonator 25. The resonators 2| and 2 are electrically connected with their outside surfaces at ground potential, the region 24 being shielded from the fields within the resonators 2| and 25.

Two parallel grids 9 and I0, forming part of the resonator 2|, are disposed in the path of travel of the electron stream. Between these grids 9 and HI, the electrons are subjected to the action of an alternating electromagnetic field produced by oscillations within this resonator excited from a suitable source by the coupling loop 54. The source may be an oscillator of the type described in Patent No. 2,242,275. After passing through the space-resonator 2| and through the field-free space 24, the electrons enter the second space-resonator 25 through an entrance grid 21 and travel thereafter through this space-resonator 25, through an exit grid |2 to an anode 56 maintained at a suitable potential.

' 26, of the resonators 2| and 25.

If desired, anode 56 may be omitted and electrode half-cycles, in accordance with the initial velocity of the entering electrons, and as determined by the resonant frequency of the space-resonant device 2|.

Each of the resonators 2| and 25 is shown as reentrant, as described in a copending application of William W. Hansen and David L. Webster, Serial No. 220,414, filed July 21, 1938, now Patent No. 2,227,372, granted December 31, 1940, the front reentrant poles being shown at 22 and 26, and the rear reentrant poles at 23 and 29. The resonators have substantially the exterior shape, roughly, of a toroid, as indicated in Fig. 2, except for the fact that the reentrant poles do not meet at, but are truncated before reaching, the respective centers of the resonators.

The grids 9, Ill and 21, I2 may be in the form of multiple-apertured conducting members, as shown. The grids 9, I9 and 21 are designed so as to offer as little obstruction to the passage of the electron stream as possible and are located at the inner extremities of the poles 22, 23 and The grids 9, I 0 and 21, I2 serve to confine the resonator standing fields within the respective resonators. The electron stream, therefore, is projected through the pole 22 and grid 9 before entering resonator 2| through the grid II) and the pole 23 after leaving it, through pole 25 and grid 21 before entering the resonator 25, and through the grid l2 and pole 29 after leaving the resonator 25.

Energy may be transferred to or from the electromagnetic field of a resonator of this character by means of inductive loops or capacitive elements in the field as well as by means of a stream of electrons. In the device of Fig. 1, an i'nput'coupling'loop 54 is shown which may be coupled to or constitute a receiving antenna or other source of high frequency energy, and ex- 10 cites the resonator 2| to oscillation. The degree magnetic fields of these space-resonators, which comprise sustained standing waves, are contained entirely within, so as to be bounded by, the conducting Walls of the respective space-resonators. All conduction currents are on the inner surfaces of these walls. The portion of the field of the resonator 2| that coacts with the electron stream extends between the opposed inner surfaces of the grids 9 and Ill. The portion of the field of the resonator 25 that coacts with the electron stream extends between the opposed inner surfaces of grids 2'! and I2. 'Except for the openings in the grids 9, l9 and 21, I2, these resonators constitute substantially closed and non-radiating containers.

The oath-ode structure 4 should be so designed and the accelerating electrode 20 so shaped that the stream of electrons shall be focused into a collimated beam that does not spread excessively, notwithstanding that it is projected a considerable distance. The form of the electrode 2|] depends upon the physical configuration of the apparatus. Though it is connected through the frame of the apparatus and ground to the positive terminal of the accelerating battery 6, it does not take part in the hereinafter-described high of insertion of this loop into the resonator controls the coupling. A similar output coupling loop 55 is supplied for resonator 25, entering the interior of resonator 25 through a coupling slot, as in the case of loop 54. The portion of loop 55 remaining outside the resonator may constitute the radiator or load circuit or be coupled to such load circuit.

It will first be assumed that the electrons emitted from the electron source 4 are caused to travel by the accelerating voltage 6 toward the grid 9 in the form of a beam which enters this grid 9 with uniform velocity so as to provide a uniform distribution in time of the electrons in the electron stream. This uniform entering velocity will hereinafter be referred to as the normal or mean velocity of the electrons. Upon passing through the grid 9, the electrons enter the space-resonant device 2|, in which the beforedescribed alternating electromagnetic field has been set up by a signal of the same frequency that it is desired to multiply, to which the spaceresonant device 2| is substantially tuned. This signal may be introduced through the coupling input loop 54.

The alternating electric field thus set up in this resonator 2| by the input signal results in imparting periodic or cyclical increments and decrements of energy, of the period of the field of the resonator, to the electrons entering through the grid 9 and traveling through the field until they exit through the grid I0. According to the direction of the electric field, some of the electrons, as they exit, will have become accelerated and their velocities will thus have become increased in varying amounts; and others.- will have become decelerated and their velocities decreased in varying amounts. The net speed of still others of the electrons will re-- main unchanged; these electrons will retain their-normal or mean velocity. The successive accelerations and decelerations of the electrons whose velocities become changed are periodic, and at the frequency of the input to space,- resonator 2| which is substantially the same as its resonant frequency. The speed of a particular electron at the time that it arrives at the grid l0, therefore, may be either higher than, equal to, or lower than the uniform or mean velocity of the stream. Some of the electrons of the stream, as they emerge from the grid Ill, will have the meanvelocity of the stream, the velocities of others will be higher than this mean velocity, and the velocities of still others will be lower. The electrons will thus assume periodically or cylically varying velocities, in synchronismwith the frequency of the field of the resonator 2| as they leave the grid III. In the absence of other forces, the electrons will continue to travel beyond the grid ID at whatever speed they had when they left this grid. They will thus travel throughthe space,

24 between the grids llland 21 (which is free from alternating fields) with unchanging individual velocities, but some with higher velocities and others with lower velocities than the mean velocity of the electrons. In the space 24, in other words, the periodic increments and decrements of energy and, consequently, the relative velocities of the electrons, will bemaintained substantially constant. If permitted thus to travel for a suitable time, the electrons will no longer be distributed uniformly in time, as they were when they entered the space between the grids 9 and Ill. The slow electrons will not be so far from the grid ID as they would have been if they had retained their normal velocity, and the fast electrons will similarly be farther from the grid l0 than they would have been if they had continued to travel at normal velocity.

If an electron that has been decelerated during its travel through the space between-the grids 9 and I0 passes the grid l0 at one instant, followed the next instant by an electron that has become accelerated during its passage through the'same space,'the faster electrons will gain on the slower electrons during their traveltoward the anode 56. A normal-velocity electron will thus, in time, catch up with those electrons, of reduced velocities, that emerged'from the grid l0 beforeit. The electrons that leave the grid i0 later than this normal-velocity electron, with higher velocities, will similarly catch up with this normal-velocity electron. If the time of travel of the electrons in the space 24 is large compared to the period of the field of the resonator 2|, therefore, it will be possible, at a particular instant of time, to find along the axis of the space 24 regions in which the electron density is somewhat increased and regions where it is somewhat diminished.

The distance between the grids l0 and 21 may be such that the electrons of mean velocity may require one or more, such as one-and-one-half or two, cycles of the frequency ofv the resoriator field to traverse it. In such cases, tlie distance traveled by the electrons will be large enough so that the electrons become simultaneously concentrated in the space 24 intov a plurality of groups or bunches of different degreesof con 'by the time interval elapsed since leaving the grid ID, the degree of excitation of the field between the grids 9 and ill, and the magnitude of the voltage source 6.- Between successive groups,

there will correspondingly be found a number of positions where there will be the electron distribution.

If an observer could visualize these groups of electrons between the grids 0 and 21, he would see one such group concentration pass a particular point of observation for each cycle of' the alternating electric field between the; grids 8 and I0. Greater increments and decrements 'of' energy are imparted to the electrons individually while in the resonator v25 than were imparted to them in the resonator 2|. However, in resonator 25, decrements are applied to more electrons than increments.

By utilizing these periodic changes in the electron density of thestream entering the resonator 25, it is possible to absorb energy from the successive concentrations of electons in the bunched or grouped stream, by deliveringthis energy, in the form of pulses, to an alternating electric field at the natural frequency thereof. The groups can thus ,be made to excite the space-resonant circuit 25 of Fig. 1,if properly positioned to receive the groups, or to create or build up and maintain the alternating electric field therein. The concentrations of theseelectron groups passrarefactions in ing through the space between the grids;2'|-|2 of. the resonator-25 of Fig. 1 is therefore equivalent, when properly utilized, to an alternating current. 1

The time of arrival of a group at grid-21 is substantially the time of arrival of the normal or mean velocity electrons. If it requires a halfcycle for the groups to traverse the space between the grids 2'I--|2, for example, and the group is to deliver maximum energy, it should arrive at the grid 21 when the field between the grids 21 and I2 is zeroand starting to oppose the motion of the electrons.

The manner in which amplification of the signal is obtained will now be described. a v

It will be assumed.v that the tuned element 25 is adjusted so that it resonates at. the. frequency of the input signal voltage. The electrons, when they reach a plane, say, halfway between the grids 2Iand l2, therefore, have'ag-degree of bunching .or grouping determined by the voltage ofthe signal source acting on the resonant element 2| through the loop 54. If the alternating field between the grids 21 and I2 were absolutely zero, the groups of electrons would do. no-work to excite the resonant circuit 25. The condition of no field between the grids 21 and 12 never, however, exists in practice. actually at least a minute field, caused by thermal agitation of the electrons in the resonantcigcuit 25. This minute field, since it originates from random motions, will take'on all possible phases with respect to the phase of arrival of the bunches of electrons in the interspace between the grids-21' tronsduring their passage between thefgrids z l and l2, these groups of-electrons will; become sloweddown. Thekinetic energy thusabsorbed There is always from these electron groups will become stored as energy of oscillation in the resonator 25. The field, will therefore, become enhanced. This increase in the strength of the oscillating field in the resonator 25 will then oppose more strongly the subsequent groups of electrons, resulting in still further enhancing the strength of the field and the magnitude of the oscillations in the resonator 25 until the circuit losses of the resonator 25 become equal to the energy supplied to the resonator 25 by the grouped electron beam. In an efficient resonator 25, the losses will not equal the energy supplied by the groups until the field in the resonator 25 is much greater than the field producing the grouping, thereby producing amplification. This amplifier device is claimed in the parent Patent No. 2,242,275.

The field'of resonator 25 will be directed half the time so as to increase the energy of any electron passing therethrough at such time. Some of the energy in the resonator 25 will thus .be returned to the electrons moving along between the bunches, but as these electrons are less numerous, and as they receive an increase in energyper electron that is substantially the same as the decrease in energy per. electron suffered by the groups or bunches, the total energy lost to the electrons by the field will be less than the energy gained'fromthe electrons by the field. A substantial portion of the energy of a group may in this way be given up to the field of the second space-resonator 25.

On the other hand, a negligible amount of energy is given to the electron-stream by the signal source 54 if the maximum cyclic voltage developed .by the signal between the grids 9 and H) of the resonator 2| is sufficiently small compared with the electron-beam accelerating voltage of battery 6. Even though this signal modulating voltage between the grids 9 and I0 is extremely small compared with that of the source 6, the electrons will be able to group themselves to some extent in the space beyond the grid III, as already described despite the negligible energy required to accomplish this effect.

In the foregoing discussion, it has been assumed, merely for convenience of explanation, that the time spent by the electrons in the interspace between the grids 9 and I0 is the same regardless of whether they are being accelerated or decelerated. This assumption is approximately fulfilled if the change of velocity between cillatory field between the grids 9 and ill will produce electron grouping that may produce an oscillatory field between the grids 2! and I2 greater than, or amplified with respect to, that existing between the grids 9 and I0. If sufficient energy is returned from the resonant circuit 25 to the resonant circuit 2|, sustained oscillations will result. If the energy returned from the resonant circuit 25 to the resonant circuit 2| is.

almost, but not quite, sufficient to sustain oscillations, however, the losses in the resonant circuit 2| will be almost cancelled by the returned energy. If a very small external source of oscillating energy is supplied to the resonant circuit 2|", therefore, a relatively large amplitude of oscillations will be produced between the grids 9 and HI.

In the case of an amplifier in which there is no feedback between the electron-grouping circult and the circuit absorbing energy from the groups, the latter circuit will automatically take on that phase relation to the incoming groups that will extract power from the incoming groups.

Analysis shows that these groups of electrons entering the output resonator 25 contain an extremely high component of harmonic frequencies bearing an integral multiple relation to the fundamental frequency. Resonator 25, however, has a very high ratio of stored energy to loss of energy per cycle, and its resonant frequencies in diiferent modes are not integral multiples of one another. Therefore, only one mode will have a strong excitation from the beam, which is the mode having a resonant frequency corresponding to the frequency of a component of the electron beam current.

In operating as an amplifier, as described above, resonator 25 is of substantially the same resonant frequency as resonator 2|, and accordingly extracts energy from the electron stream at the fundamental frequency of the electron stream current. However, if resonator 25 is tuned to one of the harmonic frequencies of the electron stream current, it will absorb energy from the electron groups at the corresponding multiple of the frequency of the periodic velocities imparted to the electrons in resonator 2|.

The apparatus therefore serves as an eflicient frequency multiplier device, since the output energy derived from the coupling loop 55 of output resonator 25 is then an integral multiple of the energy supplied to the input coupling loop 2|.

One actual preferred construction of the confined field resonators 2| or 25 is shown in Fig. 2, having radial slots 35 in the resonator wall for the purpose of making the resonator slightly resilient, so that the output resonator 25 may be tuned to the input resonator 2| or to a harmonic thereof by changing the spacing between grids 21 and I2. This is accomplished in the tube of the present invention by scaling to the end of glass envelope a metal bellows 32, and to the end of this bellows a tuning rod 34, which may be moved to exert pressure against grid I2 by micrometer nut 33 bearing against the stationary bearings 36. The slots 35 in resonator 25 may be very narrow, in which case they will not radiate appreciable amounts of energy.

It is interesting to note that in the type of tube described, the only exterior connection necessary is the connection of battery 6 to cathode 4 and the resonators. All other connections are within the tube. Also, if there is too great scattering of the electrons in their passage from the first grid 20 to the anode 56, magnetic or electrostatic focusing can be readily applied to insure that all electrons passing the first grid 20 will either strike or pass through the grid 21.

I have further found that in order to prevent too great obstruction and scattering of the beam during passage through the grid, the first grid may be made of the slat type with wires running in only one direction, whereas the third grid may have its wires running at right angles thereto.

Utilizing the frequency multiplier described above, it is possible to produce a large amount of power in wavelengths of the order of ten centimeters or smaller, at which point the size of the resonators for these low wavelengths are such that they are readily included within the tube envelope. The resonant wavelengths of metal resonators such as have been described are about two and one-half times the diameter thereof.

The practical use of a frequency multiplier such as I have described is extremely simple. The output circuit 25 may be readily tuned to the desiredharmonic of input circuit 2| simply by varying the distance between the grids 21 and I2 through the medium of the nut 33. A corresponding deformation of the field of resonator 2| may be effected to adjust the frequency thereof. There is therefore provided a frequency multiplier operating at ultra high frequencies, where none of the circuits carrying high frequencies, except the circuit carrying the input signal, is outside the tube, where all exterior surfaces of the resonant circuit members are at ground potential, wherein all the resonant circuits are completely contained within a single tube envelope, and wherein the circuits may be completely shielded from each other and from all other structures.

It should also be pointed out that there are many other forms of resonant circuits and confined field resonators that may be used to prac-' tice my invention, and the reentrant type of confined field resonator has been described herein as one preferred circuit embodying my invention.

For example, as shown in Fig. 5 of parent Patent No. 2,242,275, resonators of the form of figures of revolution cooperating with electron streams of annular cross-section may be utilized to provide greatly increased power at such ultra high frequencies.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An alternating current harmonic generator comprising an electron emitter, means aligned with said emitter for accelerating the emitted electrons and forming them into a stream, a source of frequency to be multiplied, means coupled to said source for producing an alternating electric field along the path of said electron stream, mutually spaced circuit elements located along said path spaced from said alternating field whereby grouping of electrons is effected after the stream emerges from said alternating field, and circuit means coupled to said elements for causing the said circuit elements to take high frequency electric energy from the stream of electrons at a harmonic of the frequency of said alternating field, whereby an alternating current of said harmonic frequency is generated in said circuit means.

2. High frequency apparatus comprising a first cavity resonator tuned substantially to a predetermined frequency and having a pair of electronpermeable walls defining a narrow gap therebetween, a second cavity resonator tuned substantially to a harmonic of said predetermined frequency and having a pair of electron-permeable walls defining a second narrow gap therebetween, means defining. a field-free space extending from said first gap to said second gap, a cathode adjacent said first resonator in alignment with said gaps, means aligned with said cathode for projecting electrons from said cathode through said gaps, and means coupling a source of high frequency energy of said predetermined frequency to said first resonator, whereby energy of a harmonic of said predetermined frequency may be extractedfrom said second resonator.

3. High frequency apparatus comprising a first cavity resonator tuned substantially to a-predetermined frequency and having a pair of electron-permeable walls defining a gap therebetween, a second cavity resonator tuned substantially to a harmonic of said predetermined frequency and having a pair of electron-permeable walls defining a second gap, a cathode in alignment with said gaps, means aligned with said cathode for projecting lectrons from said cathode through said gaps, and means couplinga source of high frequency ener y f said predetermined frequency to said first resonator, whereby energy of a. harmonic of said predetermined fre-, quency may be extracted from said second resonator. g 4. Ultra high frequency apparatus for multiplying the output frequency of a source of predetermined frequency, comprising means for producing an electron stream, a cavity resonator coupled to said source and in the path of said stream for periodically varying the velocities of the electrons thereof at said predetermined frequency and a second cavity resonator spaced from said first resonator by a field-free space and tuned to a harmonic of said predetermined frequency, whereby ultra high frequency energy of said harmonic frequency is excited within and may be abstracted from said second resonator..

5. Ultra highfrequency frequency-multiplier apparatus comprising a pair of cavity resonators having respective pairs of electron-permeable walls, said pairs being separated by a field-free space, a cathode aligned with said walls, means aligned with said cathode for projecting a stream of electrons from said cathode successively through one pair of said walls, said space and the other pair of said walls, said first resonator being tuned to a frequency to be multiplied, and said second resonator being tuned to a harmonic of said frequency, whereby, upon excitation of said first resonator from a source of a frequency to be multiplied, harmonic frequency energy may be extracted from said second resonator.

6. Ultra high frequency apparatus comprising a source of ultra high frequency energy of predetermined frequency, means for producing an electron stream, a pair of electrodes along the path of said stream, means coupling said source to said electrodes to produce an electric field of said predetermined frequency in the direction of said path, a second pair of electrodes positioned along the path of said stream and spaced from said first pair, means defining a field-free space between said two pairs of electrodes, and a tuned circuit tuned to a harmonic of said predetermined frequency and coupled to said second pair of electrodes for extracting ultra high frequency energy of said harmonic frequency from said stream.

7. Ultra high frequency apparatus comprising means for producing an electron stream, a pair of electrodes along the path of said stream,' a second pair of electrodes spaced from said first pair along said stream, a source of ultra high frequency energy of a predetermined frequency to be multiplied coupled to said first pair of electrodes, and a load circuit tuned to a harmonic of said predetermined frequency coupled to said second pair of electrodes, whereby ultra high frequency energy of said predetermined frequency derived from said source is multiplied by said apparatus.

8. Ultra. high frequency frequency-multiplying apparatus comprising means for producing an electron stream, a source of ultra high frequency energy to be multiplied, means along said stream path coupled to said source and in energy inter changing relation with said stream for periodically varying said stream at said predetermined frequency and harmonics thereof, and further means along said path in inductive energy interchanging relation with said stream for extracting ultra high frequency energy of a desired harmonic of said predetermined frequency from said stream.

9. Apparatus as in claim 8 wherein said lastnamed means comprises a pair of electrodes positioned-a1ong the path of said stream and a circuit tuned substantially to said harmonic frequency and coupled to said electrodes.

10. Ultra high frequency frequency-multiplying apparatus comprising a source of ultra high frequency energy of a predetermined frequency to be multiplied, means for producing an electron stream, means coupled to said source for periodically varying the velocities of the electrons of said stream at said predetermined frequency, and means for extracting from said velocity-varied stream ultra high frequency energy at a harmonic of said predetermined frequency.

11. Apparatus as in claim 10 wherein said lastnamed means comprises a pair of electrodes positioned along said stream and a tuned circuit tuned to said harmonic frequency and coupled to said electrodes.

12. Apparatus as in claim 19 wherein said lastnamed means comprises a cavity resonator having a pair of closely adjacent electron walls positioned along the path of said velocity-varied stream and in energy-interchanging relation thereto, said resonator being tuned substantially to said harmonic frequency.

13. High frequency apparatus comprising means for producing a stream of electrons, a source of fundamental frequency wave energy, an electron velocity-modulator along the path of said stream and coupled to said source to be excited therefrom, an energy-extractor separated from said velocity-modulator, and means defining a field-free drift-space between said modulator and said extractor, said extractor including a resonant circuit tuned substantially to a harmonic of said fundamental frequency, whereby the output energy derived from said extractor is of a frequency harmonically related to that of said SOHI'CB.

RUSSELL H. VARIAN.

REFERENCES CITED The following references are of record in the file of this patent:

v UNITED STATES PATENTS Number 

